The present invention generally relates to an output circuit used in circuits such as a level converter circuit, logic circuit, and an operational amplifier circuit, and, more particularly, to an output circuit for outputting an output signal having an amplitude that exceeds the breakdown voltage of a transistor of the output circuit.
In the past, an output circuit equipped with a CMOS inverter circuit was driven by receiving power from a high potential power supply Vdd (for example, 5 volts) and a low potential power supply Vss (for example, 0 volts). An input signal is amplified in a full scale within the range of power supply Vdd or Vss levels. An inverse signal of the input signal is output from the output terminal of the inverter circuit.
Due to miniaturization of semiconductor integrated circuit devices in recent years, MOS transistors with a lower breakdown voltage than the power supply Vdd or Vss level are used. However, it is desirable to provide an output signal amplified in a full scale within the range of power supply Vdd or Vss level. Accordingly, the MOS transistor of the output circuit should have a breakdown voltage that exceeds the differential voltage between the power supplies Vdd and Vss. For this purpose, special MOS transistors with a high breakdown voltage are used for the output circuit. Specifically, the high breakdown voltage MOS transistor has a relatively thick gate insulating film formed by repeating a gate oxide film formation process during fabrication.
However, repeating the gate oxide formation process increases the dispersion of MOS transistor characteristics and raises the manufacturing costs of the devices. Moreover, because the MOS transistor with a thickly filmed gate insulating film has a relatively high threshold, it also has an increased on resistance. Consequently, the drive capacity of the transistor is decreased. In order to increase the drive capacity of the transistor, the size of the transistor can be increased. However, increasing the transistor size prevents high integration of semiconductor integration circuit devices.
It is an object of the present invention to provide an output circuit that outputs an output signal having an amplitude exceeding the breakdown voltage of the MOS transistors from which it is constructed.